BEIJING - Scientists have discovered what might be the main factor slowing down the rotation speed of the mysterious pulsars.

The Chinese-led research provides important clues to the spin-down mechanism and the magnetic field structure of pulsars, highly magnetized neutron stars born from supernova explosions of massive stars, and the densest stars known to exist.

The international team led by Ge Mingyu and Lu Fangjun, of the Institute of High Energy Physics of the Chinese Academy of Sciences, found the pulsar wind nebula (PWN) surrounding pulsar B0540-69, located in the Large Magellanic Cloud, a dwarf galaxy neighboring our Milky Way, brightened gradually after the star saw a sudden spin-down rate.

Though they typically have a radius of about 10 km, pulsars can have a mass about twice that of the Sun. Their surface magnetic field could be 2 trillion times stronger than the earth's magnetic field.

In addition, pulsars might be particle accelerators with the highest energy known in the universe.

With their super strong gravitational and electromagnetic fields and high density, pulsars are regarded as natural laboratories of extreme physical conditions. Scientists could observe many phenomena on them that they cannot replicate on Earth.

In December 2011, astronomers found the spin-down rate of B0540-69 suddenly increased by 36 percent and then remained almost constant. The cause was a mystery.

Using data obtained by X-ray astronomical satellites, they found that over 400 days, the X-ray PWN around the pulsar gradually brightened by around 32 percent, comparable to the sudden increase of the spin-down rate.

Because the PWN is powered by the pulsar wind mainly consisting of electrons and positrons and moving almost at the speed of light, a natural explanation to the brightening is the increase of the pulsar wind strength. As the wind production is further connected to the magnetic field in the pulsar magnetic pole region, the sudden spin-down rate transition was most likely resulted from a sudden enhancement of the magnetic field in that region, Lu Fangjun said.

It is the first time that PWN brightening has been observationally connected with the pulsar spin-down rate transition, implying that the pulsar wind is the main factor slowing down the pulsar spin, said Lu.

The discovery was published in the Nature Astronomy online on Monday.